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A measurement of primordial non-gaussianity will be of paramount importanceto distinguish between different models of inflation. Cosmic microwavebackground (CMB) anisotropy observations have set unprecedented bounds on thenon-gaussianity parameter f_NL but the interesting regime f_NL <~ 1 is beyondtheir reach. Brightness-temperature fluctuations in the 21-cm line during thedark ages (z ~ 30-100) are a promising successor to CMB studies, giving accessto a much larger number of modes. They are, however, intrinsically non-linear,which results in secondary non-gaussianities orders of magnitude larger thanthe sought-after primordial signal. In this paper we carefully compute theprimary and secondary bispectra of 21-cm fluctuations on small scales. We usethe flat-sky formalism, which greatly simplifies the analysis, while stillbeing very accurate on small angular scales. We show that the secondarybispectrum is highly degenerate with the primordial one, and argue that evenpercent-level uncertainties in the amplitude of the former lead to a bias oforder Delta f_NL ~ 10. To tackle this problem we carry out a detailed Fisheranalysis, marginalizing over the amplitudes of a few smooth redshift-dependentcoefficients characterizing the secondary bispectrum. We find that thesignal-to-noise ratio for a single redshift slice is reduced by a factor of ~5in comparison to a case without secondary non-gaussianities. Setting asideforeground contamination, we forecast that a cosmic-variance-limited experimentobserving 21-cm fluctuations over 30 < z < 100 with a 0.1-MHz bandwidth and0.1-arcminute angular resolution could achieve a sensitivity of orderf_NL[local] ~ 0.03, f_NL[equilateral] ~ 0.04, and f_NL[orthogonal] ~ 0.03.

Primordial non-gaussianity from the bispectrum of 21-cm fluctuations in the dark ages